sdcm/tester: check n_loader_nodes/n_loader
sdcm/tester: check n_loader_nodes/n_loader

If we set n_loaders to zero in yaml config file, following error will be raised,
so add a checking in early stage of init resources.

Traceback (most recent call last):
  File "/home/amos/scylla-cluster-tests/sdcm/tester.py", line 132, in setUp
    self.init_resources()
  ...
  File "/usr/lib/python2.7/site-packages/botocore/client.py", line 552, in _make_api_call
    raise ClientError(parsed_response, operation_name)
ClientError: An error occurred (InvalidParameterValue) when calling the
 RunInstances operation: Minimum instance count must be greater than zero

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests